1. Field of the Invention
This invention relates to screen printing presses and more particularly, to drive mechanisms for driving a screen printing carriage and/or a cylinder in such presses.
2. Description of The Prior Art
The commonly-used drive for such screen printing presses includes a rotating crank mechanism which is driven by a motor with the crank mechanism providing a drive for a lever subassembly, which, in the case of a cylinder press, is connected to the cylinder and the screen printing carriage and which, in the case of a web-printing press, is connected to the screen printing carriage to reciprocate the same. A particular problem with the crank drive is that provides a harmonic motion with a substantially equal percentage of the time, being used for acceleration of the screen printing carriage and for the deceleration thereof prior to reversing the direction of travel of the screen printing carriage. To stop the carriage requires overcoming its considerable inertia. When this inertia is not smoothly overcome as when bringing the screen printing carriage against a positive stop at the beginning of print position, there may be a banging or other hitting of the screen carriages against a fixed stop which may cause the screen not to be properly registered with respect to a prior image or to specific spot on the web.
With a crank or with a conventional cam used to drive a cylinder or web press, little has been accomplished in controlling the inertia and or for increasing the time period for deceleration of the traveling printing screen mechanisms.
The present invention is directed to providing lower inertia and a greater time period for slowing down and stopping the travel of screen printing carriage and printing means in web and cylinder presses.
To these ends, the present invention is directed to providing a controlled inertia with a profiled cam drive for cylinder or web screen printing presses in which the inertia is calculated and is controlled by minimizing the velocity, particularly when stopping the movement of the screen printing carriage in one direction and just prior to its reversing its direction of travel in the opposite direction. This is achieved by providing a faster acceleration from a stopped position over a shorter period of time than with a crank and then, providing for a much longer time and a much slower movement than with a crank, resulting in a reduced inertia for the travel of the printing mechanisms when they are nearing the end of their travel in one direction, such as coming against a register stop at the beginning print position in the web press. Additionally, cranks are used without any attempt to control the rate of change in velocity over different portions of the crank's rotations and to limit the change in velocity to more uniform incremental changes.
In accordance with another important aspect of the invention, there is provided in this drive for either a web printing press or a cylinder screen printing press, an adjustable stroke mechanism whereby the length of the stroke provided by the drive may be easily adjusted to change the amount of movement of for example, the printing cylinder and the screen printing carriage so that the same may be set for shorter printing strokes for shorter printed areas and may be set for longer printing strokes for longer printing areas. The particular adjustments are of use in also avoiding the overworking of the ink which is caused when only a short area is actually printed but the squeegee moves through a much longer printing stroke which causes the ink on the printing screen to be worked when there is no printing taking place. Preferably, the stroke is adjusted so that it is only slightly longer than the area to be printed to avoid the overworking of the ink and to maximize the speed of the printing operation. The present invention provides a relatively simple and accurate adjustment, which can be made manually or with a motorized control.
Web screen presses often have additional functional devices driven by a common drive which includes the motor-driven crank. Typical, functional devices, such as additional screen printing means, a sheeter for cutting sheets, and/or a die cutter for die cutting sheets, are driven and stopped by electrically controlled devices to assure their registration with one another. Because of the high inertia and mechanical interconnections used, it was not possible to have mechanical interconnections between the screen printing means and these other functional devices and operate at high speeds and with the good registration needed. Hence, electrical sensors and controls were used with conventional web screen printing presses to achieve the necessary registration. The present invention provides lower inertia and slower stopping of these functional devices and compliance means to allow each functional device to be brought against a fixed mechanical stop to provide a mechanically controlled web printing press.
Accordingly, the general object of the present invention is to provide a new and improved drive for screen printing presses of the foregoing kind.
Another and more specific object of the invention is to provide an improved drive having improved inertia and displacement characteristics relative to the harmonic accelerations and decelerations from a crank drive.
Another object of the invention is to provide the drive mechanism with a new and improved stroke adjustment mechanism for screen printing presses of the foregoing kind.